This invention generally relates to coating systems for protecting metal substrates. More specifically, the invention is directed to a diffusion barrier layer disposed between a superalloy substrate and a protective coating for the substrate.
Metal components are used in a wide variety of industrial applications, under a diverse set of operating conditions. As an example, the various superalloy components used in turbine engines are exposed to high temperatures, e.g., above about 750C. Moreover, the alloys may be subjected to repeated temperature cycling, e.g., exposure to high temperatures, followed by cooling to room temperature, and then followed by rapid re-heating. These components thus require coatings which protect them against oxidation and corrosion attack.
Various types of coatings are used to protect superalloys and other types of high-performance metals. One type is based on a material like MCrAl(X), where M is nickel, cobalt, or iron, and X is an element as described below. The MCrAl(X) coatings can be applied by many techniques, such as high velocity oxy-fuel (HVOF); plasma spray, or electron beam-physical vapor deposition (EB-PVD). Another type of protective coating is an aluminide material, such as nickel-aluminide or platinum-nickel-aluminide. Many techniques can be used to apply these coatings. For example, platinum can be electroplated onto the substrate, followed by a diffusion step, which is then followed by an aluminiding step, such as pack aluminiding. These types of coatings usually have relatively high aluminum content as compared to the superalloy substrates. The coatings often function as the primary protective layer (e.g., an environmental coating). As an alternative, these coatings can serve as bond layers for subsequently-applied overlayers, e.g., thermal barrier coatings (TBC""s).
When the protective coatings and substrates are exposed to a hot, oxidative, corrosive environment (as in the case of a gas turbine engine), various metallurgical processes occur. For example, a highly-adherent alumina (Al203) layer (xe2x80x9cscalexe2x80x9d) usually forms on top of the protective coatings. This oxide scale is usually very desirable because of the protection it provides to the underlying coating and substrate.
At elevated temperatures, there is often a great deal of interdiffusion of elemental components between the coating and the substrate. The interdiffusion can change the chemical characteristics of each of these regions, while also changing the characteristics of the oxide scale. In general, there is a tendency for the aluminum from the aluminum-rich protective layer to migrate inwardly toward the substrate. At the same time, traditional alloying elements in the substrate (e.g., a superalloy), such as cobalt, tungsten, chromium, rhenium, tantalum, molybdenum, and titanium, tend to migrate from the substrate into the coating. (These effects occur as a result of composition gradients between the substrate and the coating).
Aluminum diffusion into the substrate reduces the concentration of aluminum in the outer regions of the protective coatings. This reduction in concentration will reduce the ability of the outer region to regenerate the highly-protective alumina layer. Moreover, the aluminum diffusion can result in the formation of a diffusion zone in an airfoil wall, which undesirably consumes a portion of the wall. Simultaneously, migration of the traditional alloying elements like molybdenum and tungsten from the substrate into the coating can also prevent the formation of an adequate protective alumina layer.
A diffusion barrier between the coating and the substrate alloy can prolong coating life by eliminating or greatly reducing the interdiffusion of elemental components, as discussed above. Diffusion barrier layers have been used for this purpose in the past, as exemplified by U.S. Pat. No. 5,556,713, issued to Leverant. The Leverant patent describes a diffusion barrier layer formed of a submicron layer of rhenium (Re). While such a layer may be useful in some situations, there are considerable disadvantages as well. For example, as the temperature increases, e.g., the firing temperature for a turbine, interdiffusion between the coating and the substrate becomes more severe. The very thin layer of rhenium may be insufficient for reducing the interdiffusion. A thicker barrier layer of rhenium could be used, but there would be a substantial mismatch in CTE (coefficient of thermal expansion) between such a layer and a superalloy substrate. The CTE mismatch may cause the overlying coating to spall during thermal cycling of the part. Moreover, rhenium can be oxidized rapidly, which may also induce premature spallation of the coating.
It should thus be apparent that new barrier coatings which overcome some of the drawbacks of the prior art would be welcome for high-temperature metal substrates. First and foremost, the barrier coatings should have relatively low xe2x80x9cinterdiffusivityxe2x80x9d for aluminum and substrate elements. The barrier coatings should also be chemically compatible with the substrate alloy and any protective coating for the substrate. They should also be chemically and compositionally stablexe2x80x94especially during anticipated service lives (e.g., for turbine airfoils) at temperatures of greater than about 750C. Moreover, the barrier coatings should exhibit a relatively high level of adhesion to both the substrate and the protective coating. The barrier coatings should also exhibit only a minimal CTE mismatch with the substrate and coating. Furthermore, the barrier coating should be capable of deposition by conventional techniques, such as plasma spray, physical vapor deposition, sputtering, and the like.
The needs described above have been addressed by the discovery of a barrier coating material, comprising:(a)about 15 atom % to about 95 atom % chromium; and(b)about 5 atom % to about 60 atom % of at least one element selected from the group consisting of rhenium, tungsten, ruthenium, and combinations thereof.
The barrier coating material often includes other constituents as well. For example, it may include about 1 atom % to about 35 atom % of at least one element selected from the group consisting of nickel, cobalt, iron, and combinations thereof. It can also include about 1 atom % to about 35 atom % aluminum. Many of the factors involved in the selection of the composition of the barrier coating material are described below.
Another embodiment of the invention is directed to an article for use in a high-temperature, oxidative environment. The article includes a metal-based substrate (e.g., a superalloy), containing aluminum and other alloy elements, and an oxidation-resistant coating. Exemplary oxidation-resistant coatings are described below, e.g., aluminide materials, MCrAI(X) materials, and nickel-chrome materials. A barrier coating is disposed between the substrate and the oxidation-resistant coating.
The barrier layer performs several important functions. When the overlying oxidation-resistant coating is aluminum-rich, the barrier layer prevents the substantial migration of aluminum from such a coating into the substrate. (As used herein, an xe2x80x9caluminum-richxe2x80x9d coating is defined as one having a concentration of aluminum higher than the concentration of aluminum in the substrate. When comparing comparative, cross-sectional areas of the coating and the substrate, the concentration of aluminum in the coating is often about two times to about five times the concentration of aluminum in the substrate, prior to any heat treatment.).
The barrier layer also prevents the substantial migration of various substrate elements into the coating. In this manner, the integrity and service life of the coating system and the underlying substrate (e.g., a turbine airfoil) is significantly enhanced. As used herein, the xe2x80x9cprevention of substantial migrationxe2x80x9d of aluminum from an aluminum-rich coating into the substrate refers to the amount of migration which occurs during anticipated service lives for the component at temperatures of greater than about 750C. (Service lives for turbine engine components for the purpose of this explanation range from about 1000 hours to about 30,000 hours). For the present invention, less than about 10% of the aluminum migrates from the coating into the substrate, when a barrier layer is present. Very often, the amount of migration is less than about 5%. In general, the migration levels for various alloy elements (as described below) from the substrate into the aluminum-rich coating are also reduced to these levels, in the presence of the barrier layer.
Another embodiment of this invention relates to a method for preventing the substantial migration of aluminum from an aluminum-rich, oxidation-resistant coating into an underlying metal-based substrate, in a high-temperature, oxidative environment. The method includes the step of incorporating a diffusion barrier layer between the substrate and the coating. The composition of such a layer is mentioned above, and further described below. Methods for providing effective coating systems over superalloy substrates also constitute part of this invention. These methods include the deposition of the diffusion barrier layer, an overlying oxidation-resistant layer, and a ceramic overcoat, e.g., a TBC.
Further details regarding the various features of this invention are found in the remainder of the specification.